An interview with Gerold Dobler, leader of the SF design team (1 Viewer)

While travelling in Germany recently I had the opportunity to conduct an informal and unofficial interview with Herr Dobler.

We agreed that this would not be centred around the marketing claims for this binocular but would consist of me asking Herr Dobler to comment on a number of issues raised on this web site.

I will be keying out this recorded interview in the next couple of days or so and expect to be able to post a cutaway drawing of the SF optical system to accompany it during the course of next week.

Lee

Sounds great, Lee. :t:
Look forward to your interview with Mr Dobler !

Troubador said:

While travelling in Germany recently I had the opportunity to conduct an informal and unofficial interview with Herr Dobler.

We agreed that this would not be centred around the marketing claims for this binocular but would consist of me asking Herr Dobler to comment on a number of issues raised on this web site.

I will be keying out this recorded interview in the next couple of days or so and expect to be able to post a cutaway drawing of the SF optical system to accompany it during the course of next week.

Lee

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Outstanding, Lee! We are curious to learn more about that new wunderfernglas!

Cheers,
Holger

An informal and unofficial interview with
Gerold Dobler, Director, Carl Zeiss Sports Optics Product Management

Herr Dobler, thank you for making yourself available for this interview. Could you tell us a little about your career in optics leading up to you joining Zeiss Sports Optics?

I actually started to work with optical companies in 1992 / 93 as an advisor but then I was asked by Gerhard Swarovski in 1997 to join the company after working with them as a consultant. I was their first product manager. The idea was to help them build a presence in the birding market because they only had a strong connection with the hunting market and since I am a birder I could help them out with this. I was the only product manager in Swarovski at this time and worked on several projects including the EL.

I worked with Swarovski until 2008 when I joined Leica to help develop their sports optics and in particular the Magnus rifle scope range and the open bridge Perger porro Geovid. Herr Perger was behind the technical design of both the first Geovid and the new one, but for the current open-bridge model, the overall concept and industrial design was my responsibility.

In 2011 Herr Schmidt asked me to join Zeiss to re-introduce Zeiss into the birding world by developing a new birding binocular that would be better than the EL SV, and also suitable for butterfly watching and nature observation in general. It was felt Zeiss had become too concentrated on hunting and needed help to approach the nature observation market again and since I had done this for Swarovski, Herr Schmidt thought I was the right person to do this for Zeiss.

One of the key members of the SF team passed away last year. Would you tell us who he was, something about his previous work and how he contributed to SF?

One of the people who followed me from Swarovski was Konrad Seil, who did the optical design the original EL and for the Magnus scope at Leica. It was Konrad who also worked out the optical design for the SF based on my concept.

Konrad took my ideas and developed them into precise optical designs and this is what he did with SF. Several optical designers have since said that they did not think it possible to achieve what we have done with the objective lens design on SF and reach the quality of image that we have, but Konnie always took my concepts absolutely seriously and worked to make them succeed and his contribution was crucial to the success of the SF.

How does the SF optical system compare with FL & HT, and with Ultravid & EL SV

Actually the SF optical system is different from every other binocular that we know of today. The concept includes an eyepiece that adds weight to the design due to giving a wider field of view and also sharpness up to close to the edge. This weight increase is unavoidable if you want to achieve this performance and the only way to compensate for this is to take out weight from the objective system. This was the idea and the challenge.

The solution was a super-thin achromatic doublet objective weighing much less than the triplet in our FL and HT, and also the EL SV, in combination with a focusing lens that is actually designed not only as a focuser but also a critical part of the objective system. The doublet is made from glass containing calcium fluoride, different from FL and HT but still supplied by Schott and it is very expensive and a challenge to work with as it can break more easily during manufacturing.

These lenses are even thinner than those in FL and HT and brought us a very significant weight reduction which compensated for the extra weight in the eyepiece.

There has been a report of an effect in SF whereby there is an area of slight softness of the image between the centre of the field of view and the edge. What can you tell us about this?

Yes there is this effect but it will not be seen during normal viewing and it cannot be seen at all in the 8x. It is caused by apertures within the optical system, they are like field-stops so to say, and they are there to control different optical aberrations for example coma. It is always a compromise in optical systems that you improve one situation but you risk making something else worse. So we have chosen a balance between control of aberrations and the appearance of a very slight softness in a ring between the centre and the outside that will not normally be detected. By a very few people, looking very hard, in some circumstances, it is possible the effect might be seen in the 10x, but in any normal viewing by the majority of people, they would never notice it was there and even in the area where this occurs the binocular is still very competitive with other premium binoculars as far as sharpness is concerned. We are very happy indeed with result that we have got.

SF is named after the Smart Focus concept. This has given some people the impression that the focus speed should be quite fast or even that it has a dual speed focuser, but actually, although SF is faster than EL SV, it is a little slower than HT, so what is Smart about it and did you consider giving SF a dual speed focuser?

Well, it is actually possible to have focus that is too fast for most users, because you overshoot the point of focus too many times. SF is definitely a compromise but it is significantly faster than EL SV and still allows very good control of the point of focus and we are very happy with the result. We asked people in the market about dual speed focusers and most of those we asked do not like the idea of it. They just don’t want it. Also, to do it well, a dual speed focuser adds significant weight to the bins which is definitely what people do not want.

Low weight has been a priority with SF although Zeiss appears to have reacted to market pressure by moving away from the GRP body of FL to a magnesium alloy. Do you think that you have gone as far as it is possible to go with weight reduction while staying with a metal body? If so, what do you think might be the next step?

Magnesium alloy is used more and more for different applications and the supply is getting better so the price has come down a little from previous times and a full metal housing has a very strong image in the premium market and to be practical it does provide a certain ruggedness. It’s a good solution, it has a good image and a reasonable price, so I expect it will be the standard material in the premium sector for some years.

I understand your target of 800 gram maximum for SF and in that context you have done well to keep it to 780 grams. However at 173 mm long, it is 8% longer than the EL SV despite only having a doublet objective. Is this extra length due to the extra bridge?

It has several reasons. We wanted a larger open bridge area to provide more space for the hands for a good grip than some other bins. I believe the extra length is not such a disadvantage compared with ergonomic advantages. Improved ergonomics were a big factor in the concept of SF.

Now for our final question. With SF you have tried to achieve a balance between a flat field, large field of view, sharpness up to the edge of the view and minimising the well-known effect called ‘rolling ball’. How successful have you been with these four desirable attributes?

We found a good balance between these different things and especially in the control of the so-called rolling ball or globus effect that affects between 5 and 10% of people. We contacted some of these people and asked them to try different binoculars having different levels of field-flattening and we came up with a value that gave the best balance between field flatness and control of the globus effect. This means SF is not quite so flat-field at the very edge but it is nearly so, and from what those people told us almost everybody should be able to enjoy SF without noticing any globus effect.

Will the extra wide field of view also be a feature of the SF 32 mm that is surely going to follow the 42?

Well, I cannot say anything about a specification for any model that we might develop, but it is clear that a wide field of view is a successful part of the SF concept so I would expect this to be a feature of any model developed using the SF concept.

Herr Dobler, thank you very much.

Troubadour post 4,
Today the SF was launched at the Dutch Birding Asssociation and mr. Gerold Dobler was there to give two lectures and to aswer questions. In his lecture he addressed all subjects you also described in your post 4. We heard that the SF is now in its production phase and a number were sold already this afternoon, when I left 12 pieces were already sold.
Gijs van Ginkel

. Thank you Lee for your very good interview and for the very revealing insights into a binocular design.
It does show the compromises and thought processes that are needed to produce a successful product.

Thanks again.

In addition to my post 5: the drawings of the SF design show that Zeiss has chosen for an Erfle type eyepiece to obtain a wide angle eyepiece yielding a very large FOV.
Gijs

Konrad Seil and Gerold Dobler did a very good job, congratulations and appreciation for their work.

Thanks Lee for that excellent interview - informative and very professional while asking all the right questions.

You do us proud!:t:

good job! very interesting!

does this mean that Zeiss SF have pure FLUORITE crystal in the objective lens?
(like the Kowa 883)
These different glass types confuses me a bit…
but in camera lenses, pure fluorite makes the lenses lighter compared to using, ordinary ED/HD glass.

I guess it means that the CA levels will be lower than in any current binocular?

Vespobuteo said:

good job! very interesting!

does this mean that Zeiss SF have pure FLUORITE crystal in the objective lens?
(like the Kowa 883)
These different glass types confuses me a bit…
but in camera lenses, pure fluorite makes the lenses lighter compared to using, ordinary ED/HD glass.

I guess it means that the CA levels will be lower than in any current binocular?

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The statement was 'glass containing calcium fluorite' so clearly not pure fluorite crystals.

Excellent interview, much appreciated. Thank you, Troubador!

etudiant said:

The statement was 'glass containing calcium fluorite' so clearly not pure fluorite crystals.

Excellent interview, much appreciated. Thank you, Troubador!

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just noted that it's different then FL and HT, then whats the difference?
higher content of Calcium Fluorite?
Fluorite crystal is very difficult to work with as well, and expensive…
would be nice with some more info on the Ultra-FL glass,

Dobler says:

"The doublet is made from glass containing calcium fluorite, different from FL and HT but still supplied by Schott and it is very expensive and a challenge to work with as it can break more easily during manufacturing."

and some info from Takahashi web-site:

"..Other telescope manufacturers may claim that ED (Extra-low Dispersion) glass is the equivalent of fluorite or that their older designs will work as well. Unfortunately, they are not being honest. While ED and fluoro-crown lenses can achieve Abbe-coefficients approaching fluorite, they tend to absorb more light in the visible spectrum. This means that fluorite yields a brighter, higher contrast image. Leica, Zeiss, and Kowa have all gone to fluorite in their spotting scopes and telescopes to achieve the maximum performance levels their customers demand. Most of them previously used ED glass. Obviously, they know the difference between fluorite and ED. You will too. ...
"
---

"To call lenses made of fluoro crown glass fluorite lenses is misleading and is a marketing attempt to sell on the back of the outstanding performance/ reputation of telescopes which used Calciumfluorit elements such as the Takahashi FS-102, FS-128, FS-150, the Vixen 102 and Calciumfluorit doublets of current production."

http://www.iceinspace.com.au/forum/archive/index.php/t-121155.html

Lee:

Nice job with a very good and informative interview. One thing that I found
interesting was that Dobler estimated 5-10% of individuals may have an issue
with the globe effect or "rolling ball".

It seems that this topic has been well hashed on this forum, and the numbers are quite low,
compared to what has been the emphasis of it being a common problem.

Jerry

Jerry,
Gerold Dobler has read this forum very well is my conclusion on the basis of some of his remarks during this afternoons lecture, so his estimate of 5-10% of the binocular-using population which would be sensitive to the globus effect may well have come from this forum.
Gijs.

Lee, thanks a lot for this interesting interview! I agree that Herr Dobler and Herr Seil have taken the challenge of a very ambitious project, and surely something great has been achieved. Most notably an increase in field of view with, at the same time, high edge sharpness, that's something quite remarkable!

The doublet lens apparently contains one lens element made of CaF2, so it is a classical ED. These CaF2 lenses are not only hard to shape, but also difficult to coat, since not all of the anti-reflex layers are sticking well on that material. I guess this is an air-spaced doublet, with "super-thin" lens elements, one of them made of CaF2 - let's hope it won't break upon a strong mechanical impact.

I do not quite understand that explanation for the "Absam-ring", caused by a "kind of field stop" to reduce aberrations like coma. A field stop would affect the field of view, not the sharpness, and the effects of any other kind of stops should increase with the angle, thus affect the edges of field. But the Absam-ring is something that shows up in between. I would have guessed that it was a higher order field curvature effect, but anyway ...

Regarding the globe effect, note that Mr. Dobler claimed 5-10% of the users being affected by that effect. "Affected" may refer to more serious reactions than just noticing the effect. I would guess that, with distortion-free optics, at least 40% of the users would notice the effect, but certainly less than 10% are feeling seriously disturbed by that effect. It is great to know that they have taken the side effects of low-distortion serious and that they have conducted tests about them. I would be very much interested in the test results they have got, to compare them with those theoretical and simulation studies I have published a few years ago.

Cheers,
Holger

Great interview, Lee. :t:
I appreciate it, greatly !

Holger Merlitz said:

The doublet lens apparently contains one lens element made of CaF2, so it is a classical ED. These CaF2 lenses are not only hard to shape, but also difficult to coat, since not all of the anti-reflex layers are sticking well on that material. I guess this is an air-spaced doublet, with "super-thin" lens elements, one of them made of CaF2 - let's hope it won't break upon a strong mechanical impact.

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If it's "made of CaF2" do you mean it's pure fluorite?

I'm not a glass expert but I guess "Ultra-FL" could mean a higher CaFl2 content than ordinary FL-glass, but maybe not 100%?

Pure fluorite crystal is said to be more sensitive to heat, so maybe the light gray rubber armoring of the SF is an indication of that there is something crystal like in there (canons camera lenses are said to be white of the same reason.)

anyway, it will be interesting to see if chromatic aberrations
is better corrected than in the FL-series which I think is good enough, but
not perfect.

Lee,

Thanks for the interview. A good read. Interesting to hear the take of the product manager and gain some insight into their thinking.

Holger,

I share your thoughts on the "Absam ring" explanation. It would be nice to hear a bit more about it, since the field-stop explanation does not sound quite right.

Incidentally, I'm a bit annoyed by the "Absam" part of the term that has gained hold on these forums. It was probably coined by Brock in his crusade against the RB-effects of the Swarovisions, but the phenomenon it describes I first saw in Nikon's SE 10x42 which I got in 1996 and still have. It has very good edge of field sharpness, but a little bit inwards from the edge the sharpness falls off before increasing again, very much like in the Swarovisions. So, for historical correctness, credit should be given to the inventors of this anomaly and it should be called the "SE ring," unless an even earlier precedent is found. I do not know if this effect is equally visible in the 8x32 SE, since that model I have not thoroughly tested or viewed with.

Kimmo

Gijs van Ginkel said:

Troubadour post 4,
Today the SF was launched at the Dutch Birding Asssociation and mr. Gerold Dobler was there to give two lectures and to aswer questions. In his lecture he addressed all subjects you also described in your post 4. We heard that the SF is now in its production phase and a number were sold already this afternoon, when I left 12 pieces were already sold.
Gijs van Ginkel

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Gijs

My interview with Herr Dobler took place on the day before your DBA meeting so he probably had these questions in the front of his mind.

Lee

Binastro said:

. Thank you Lee for your very good interview and for the very revealing insights into a binocular design.
It does show the compromises and thought processes that are needed to produce a successful product.

Thanks again.

Click to expand...

Binastro

Thanks indeed. The more you ask questions about optical systems the more you realise that it is always a question of balancing two or more opposing requirements: improve one thing and something else gets worse. Achieving a balance is a knife-edge balancing act and someone has to decide what the optimum balance is, knowing full well that some folks like us will say the balance should have been slightly different. :smoke:

Lee

Gijs van Ginkel said:

In addition to my post 5: the drawings of the SF design show that Zeiss has chosen for an Erfle type eyepiece to obtain a wide angle eyepiece yielding a very large FOV.
Gijs

Konrad Seil and Gerold Dobler did a very good job, congratulations and appreciation for their work.

Click to expand...

Yes, indeed you are right, and when speaking to Herr Dobler, his respect and admiration for Herr Seil, who is no longer with us, is evident in everything that he says.

Lee